1. Field of the Invention
The present invention relates to a magnetic sensor, a magnetic head, and a biomagnetic sensor.
2. Description of the Related Art
Magnetoresistive sensors are known as magnetic sensors for use in thin-film magnetic recording/reproducing heads and the like. In general, a magnetoresistive sensor applies a current between a magnetization-fixed layer and a magnetization free layer and therefore provides a high output. However, the magnetoresistive sensor receives a signal, caused by the motion of a domain wall due to current-induced spin torque or the like, unnecessary for magnetic sensors.
Spin accumulation (SA) magnetic sensors each including a magnetization free layer and magnetization-fixed layer formed on the same horizontal surface (a channel layer for accumulating spins) are known (refer to, for example, Japanese Unexamined Patent Application Publication No. 2007-299467 and Japanese Patent No. 4029772). For example, when a thin-film magnetic recording/reproducing head includes a spin accumulation magnetic sensor, no current needs to be applied to a magnetization free layer detecting the external magnetic field of a magnetic recording medium or the like. That is, the spin accumulation magnetic sensor can detect magnetic condition in the form of an output voltage using a spin current only.
In order to put spin accumulation magnetic sensors to practical use, some problems need to be solved. One of the problems is noise. In the case of, for example, a magnetoresistive magnetic sensor, a current is applied to a multilayer film detecting a voltage and therefore the resistance of the multilayer film can be a cause of Johnson noise or the like. In the case of a spin accumulation magnetic sensor, a current is applied between a channel layer and a multilayer film for applying spins to the channel layer, whereby spins are injected into the channel layer. The current applied therebetween is a cause of noise. The noise of a current acts as the noise of a spin current and is detected together with output; hence, a high signal-to-noise (S/N) ratio is not obtained. A solution to this problem is to increase the cross-sectional area of a multilayer film in contact with a channel layer as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2010-113788. However, any solution to noise due to the detection side of a spin current is not described therein.
In the case of a spin accumulation magnetic head, a magnetization free layer detecting an external magnetic field needs to be smaller than a magnetization-fixed layer in order to enhance spatial resolution with respect to the external magnetic field and therefore the magnetization free layer generally has higher interfacial resistance. When the magnetization free layer, which is placed on a channel layer, and the magnetization-fixed layer have the same multilayer structure, the areal resistance of the interface between the channel layer and the magnetization free layer is equal to the areal resistance of the interface between the channel layer and the magnetization-fixed layer. Thus, in the case where the magnetization free layer is prepared so as to be smaller than the magnetization-fixed layer, the resistance of the interface between the channel layer and the magnetization free layer is higher than the resistance of the interface between the channel layer and the magnetization-fixed layer. This applies to the case where the magnetization free layer is used as an injection source of a spin current and the case where the magnetization free layer is used as an electrode for detecting a spin current. When the resistance of the interface between the magnetization free layer and the channel layer is high, there is a problem in that no high S/N ratio is obtained because Johnson noise increases in proportion to circuit resistance.